Many implantable medical devices such as pacemakers, defibrillators and neural stimulators deliver electrical therapy to tissue and sense various physiological parameters via medical leads. Such leads typically include an elongated flexible lead body with one or more electrodes disposed at a distal end of the lead. The electrodes are connected to a terminal pin on the lead's proximal end by conductors that are disposed within the lead body.
The lead is typically coupled to a header of the implantable medical device with a proximal portion of the lead being secured within the header to prevent the lead from dislodging. In general, the header has a connector block that includes a lead bore into which the lead's proximal portion is received. The connector block also includes a threaded setscrew bore that intersects with the lead bore. The setscrew bore receives a setscrew that engages the lead to secure it within the header.
The connector block is also coupled to a feedthrough pin, which passes through hermetic seals to connect with input and/or output nodes of the implantable medical device's electronic circuitry. Typically, the connector block is formed from a conductive material, such as metal, thereby permitting electrical connectivity between the lead and the electronic circuit.
To provide a reliable connection of the lead within the connector block, the setscrew is typically comprised of metal. Thus, the contact with the electrically active connector block causes the setscrew to be electrically active. Exposure of the setscrew to adjacent body tissue and body fluids might result in undesired electrical conduction to the adjacent tissue. Additionally, because the setscrew bore intersects with the terminal pin of the lead, ingress of fluid into the setscrew bore may result in the fluid contacting the terminal pin and this may compromise the device's delivery of electrical therapy. Consequently, a septum, typically referred to as a grommet, is disposed within the setscrew bore to cover the setscrew, thereby sealing the setscrew bore and isolating the electrically active setscrew from body fluids. In one example, the grommet is a silicone disk that has an elastic quality and has a slit that allows passage of a screw driver for tightening the setscrew and re-seals upon removal of the torque wrench to block entry of body fluids. Additionally, when the shank of the setscrew is disengaged from the threaded bore, the grommet retains the setscrew and prevents it from falling out.
While the use of a grommet has been satisfactory at preventing entry of fluids into the device and contact between the electrically active setscrew and surrounding tissue, it also substantially obstructs the visibility of the lead's terminal pin within the connector block. Lead tip visibility is an indicator of full lead insertion into the conductive block. The visibility enables verification that a proper and secure electrical and mechanical connection between the lead and the conductive block has been made.